Traditional IC sockets are generally constructed of an injection molded plastic insulator housing that includes stamped and formed copper alloy contact members stitched or inserted into recesses. The assembled IC socket is then generally processed through a reflow oven to attach solder balls to the contact members. During final assembly the contact pads on the printed circuit board (“PCB”) are printed with solder paste or flux and the solder balls on the IC socket are placed in registration with the contact pads. The assembly is then reflowed and the solder balls essentially weld the IC socket to the PCB.
During use, the IC socket receives an IC device, such as a packaged integrated circuit. The contact members electrically couple the terminals on the IC device with the corresponding terminal on the PCB. The terminals on the IC device are typically held against the contact members by applying a load, which is expected to maintain intimate contact and reliable circuit connection throughout the life of the system without a permanent connection. As a result, the IC device can be removed or replaced without the need for reflowing solder connections.
These types of IC sockets and interconnects have been produced in high volume for many years. As IC devices advance to next generation architectures traditional IC sockets have reached mechanical and electrical limitations that require alternate methods. For example, increased terminal count, reduction in the distance between the contacts known as terminal pitch, and signal integrity have been the main drivers that impact the IC socket design. As terminal counts go up, the IC package essentially gets larger due to the additional space needed for the terminals. As the package grows larger, costs go up and the relative flatness of the package and corresponding PCB require compliance between the contact members in the IC socket and the terminal pad to accommodate the topography differences and maintain reliable connection.
As the terminal pitch is decreased the thickness of the insulating walls in the IC socket housing is also decreased. The length of the contact members is frequently increased to optimize the spring properties. Longer contact members also tend to reduce signal integrity and increase contact resistance due to self-heating of power delivering contacts. The thinner insulating walls increase the difficulty of molding and increase latent stresses in the IC socket housing, increasing the risk of warpage during solder reflow. The thinner insulating walls also increase the risk of cross-talk between adjacent contact members.
Traditional IC sockets have reached an electrical performance limit. Next generation IC devices will operate above 5 GHz and beyond and the existing IC sockets do not provide acceptable performance levels without significant revision.
For example, traditional test sockets are manufactured from bulk plastic material that is machined to provide device location features as well as positions for the electrical contacts, which can be stamped and formed, blanked, wire electro-discharge machining processed, conductive elastomer, coil spring probes or several variations. The predominant contact type used in test sockets is the spring probe, which basically consists of two or more metal members that engage each other to create the electrical path biased by a coil spring that provides normal and return force. A major issue with the use of spring probes in test sockets is the electrical performance is degraded by the coil spring which is an inductor, as well as the potential capacitance of the metal members and the relatively high contact resistance due to the various sliding connection point.